Electroplating has many applications. One very important developing application is in plating copper onto semiconductor wafers to form conductive copper lines for “wiring” individual devices of the integrated circuit. Often this electroplating process serves as a step in the damascene fabrication procedure.
A continuing issue in modern VLSI wafer electroplate processing is quality of the deposited metal film. Given that metal line widths reach into the deep sub-micron range and given that the damascene trenches often have very high aspect ratios, electroplated films must be exceedingly homogeneous (chemically and physically). They must have uniform thickness over the face of a wafer and must have consistent quality across numerous batches.
Some wafer processing apparatuses are designed to provide the necessary uniformity. One example is the clamshell apparatus available in the SABRE™ electroplating tool from Novellus Systems, Inc. of San Jose, Calif. and described in U.S. Pat. Nos. 6,156,167, 6,159,354 and 6,139,712, which are herein incorporated by reference in their entirety. The clamshell apparatus provides many advantages in addition to high wafer throughput and uniformity; such as wafer back-side protection from contamination during electroplating, wafer rotation during the electroplating process, and a relatively small footprint for wafer delivery to the electroplating bath (vertical immersion path).
There are many factors that can effect the quality of an electroplating process. Of particular note in the context of the present invention are problems having their genesis in the process of immersing the wafer into an electroplating bath. As indicated, bubbles can be entrapped on the plating underside of the wafer (the active side) upon immersion. This is especially true when the wafer is immersed in a horizontal orientation (parallel to a plane defined by the surface of the electrolyte) along a vertical immersion trajectory. Depicted in FIG. 1A is a cross-sectional diagram of a typical bubble-entrapment scenario arising in an electroplating system 101. A horizontally oriented wafer 103 is lowered towards an electrolyte 107 in a vessel 105 along a vertical Z-axis and ultimately immersed in the electrolyte. Vertical immersion of horizontally oriented wafer 103 results in air bubbles 109 being trapped on the underside (plating surface) of wafer 103.
Air bubbles trapped on the plating surface of a wafer can cause many problems. Bubbles shield a region of the plating surface of a wafer from exposure to electrolyte, and thus produce a region where plating does not occur. The resulting plating defect can manifest itself as a region of no plating or of reduced thickness, depending on the time at which the bubble became entrapped on the wafer and the length of time that it stayed entrapped there. In an inverted (face down) configuration, buoyancy forces tend to pull bubbles upwards and onto the wafer's active surface. They are difficult to remove from the wafer surface because the plating cell has no intrinsic mechanism for driving the bubbles around the wafer edges, the only path off the wafer surface. Typically, wafer 103 is rotated about an axis that passes through its center and is perpendicular to its plating surface. This also helps to dislodge bubbles through centrifugal force, but many of the smaller bubbles are tenacious in their attachment to the wafer.
Therefore, while horizontal wafer orientation (especially coupled with a vertical immersion trajectory) has numerous advantages from a hardware configuration and throughput standpoint, it leads to technically challenging issues associated with gas entrapment and consequent defect formation.
One way to facilitate removal of entrapped bubbles is to use a vertically directed electrolyte flow aimed at the plating surface of the wafer. This can help dislodge the bubbles. As depicted in FIG. 1B, scenario 102, plating solution is directed from a conduit 111 normal to the plating surface of the wafer at a velocity sufficient to dislodge entrapped bubbles. As indicated by the arrows emanating from 111, the majority of the flow is directed at the center of wafer 103. As the flow encounters the surface of the wafer, it is deflected across the wafer surface to push the bubbles toward the sides of wafer 103 as indicated by the dashed arrows. This helps remove bubbles that are not only generated upon immersion, but also those formed or reaching the surface during electroplating. Unfortunately, the radial non-uniformity of the forced convection of such systems can result in non-uniform plating profiles. This is because the electroplating rate is a function of local fluid velocity, and the forced convection of the systems such as depicted in FIG. 1B introduces non-uniform velocity profiles across the wafer surface.
Another problem associated with vertical immersion of a horizontally oriented wafer is multiple wetting fronts. When a wafer is immersed in this way, the electrolyte contacts the wafer at more than one point, creating multiple wetting fronts as the wafer is submerged in the electrolyte. Where individual wetting fronts converge, bubbles may be trapped. Also, defects in the finished plating layer can be propagated from microscopic unwetted regions formed along convergence lines of multiple wetting fronts.
What is needed therefore is a way to improve plated metal quality. Improved methods and apparatus should reduce the problems that can arise from bubble formation and multiple wetting fronts during wafer immersion.